In medical examinations, for example, in computed tomography scans with the aid of X-rays, X-ray detectors are used. These X-ray detectors can be configured as scintillator detectors or as detectors with direct converters. In the following, X-ray detectors are taken to be any type of detector which either detects X-rays or other hard rays, such as gamma radiation.
In a detector made of a scintillator material, the scintillator material is excited on passage of the X-ray radiation and the excitation energy is output in the form of light. This visible light generated in the material is measured with the aid of photodiodes. The scintillator detector is typically configured in the manner of an array comprising a plurality of scintillator elements, wherein individual photodiodes are associated with the scintillator elements, so that the photodiodes also form an array.
Detectors with direct converters have semiconductor materials which carry out a direct conversion of the radiation incident thereupon into an electric signal. The incident X-ray radiation directly creates charge carriers in the form of electron-hole pairs. By applying a voltage (bias voltage) to the semiconductor material, the charge carrier pairs are separated by the electric field generated thereby and reach the electric contacts or electrodes which are mounted on the semiconductor material (see FIG. 1). By this means, an electric charge pulse is generated which is proportional to the absorbed energy and is evaluated by a downstream readout electronics system. Semiconductor detectors based, for example, on CdTe or CdZnTe and used in the field of human medical imaging have the advantage as compared with the scintillator detectors conventionally used there that with them an energy-sorting counting is possible, that is, depending on their energy, the X-ray quanta detected can be subdivided, for example, into two classes (high-energy and low-energy) or more.
On operation of semiconductor direct conversion radiation detectors such as, for example, detectors based on CdTe or CZT, under irradiation with gamma and X-ray radiation, particularly at high intensities, the phenomenon of polarization occurs. This is expressed in an unwanted change to the internal electric field in the semiconductor material of the detector. Due to the polarization, the charge carrier transportation properties and thus also the detector properties change. As a consequence, the pulse forms of the electrical signals corresponding to the X-ray quanta incident in the semiconductor layer, in particular the pulse height and the pulse width, are dependent to a significant degree on the polarization state of the semiconductor layer. The polarization effects are responsible for a reduction in the charge carrier mobility-lifetime product (μτ product) and therefore for an increase in the mean dwell time with simultaneous reduction of the lifetime of the charge carriers in the semiconductor material. The aforementioned polarization effects are essentially caused by defect sites in the form of vacancies or interstitial atoms during the manufacturing of the semiconductor layer. However, the polarization state of the semiconductor layer does not depend only on production-dependent parameters. It also depends on the temperature of the semiconductor material as well as on the history of the X-ray radiation impinging upon the semiconductor layer. The polarization state is thus changeable over time and is typically different between successive examinations, even before and after each individual examination.
In particular, the stated changes lead to a change in the signal properties of the measurement signal as a function of time. Expressed differently, due to the polarization, the intensity of the scan signal changes over time with a constant radiation dose. This phenomenon is known as “signal drift”. A detector is constructed from a plurality of pixels. Since the signal drift of the individual pixels is different, for the detector there is a distribution of the signal drift factors associated with the individual pixels. Over time or under irradiation, this distribution changes and the width of the distribution of the signal drift factors increases significantly more strongly than the mean value of this distribution.
A possibility for reducing the signal drift lies in using the fact that the width of the distribution of the signal drift factors grows more strongly than the mean value of the distribution. A plurality of detectors are combined into groups of individual pixels known as “macropixels”. These macropixels can comprise, for example, a number of 2×2, 3×3 or 4×4 individual pixels. In order to reduce the signal drift, individual pixels which drift strongly are entirely excluded from the signal transference. In this way, an improved drift behavior of the detector signal is achieved. However, this improvement is gained at the cost of a very great worsening in the detector efficiency, i.e. a signal usage that is worsened by, for example 6.25% to 25% and therefore also a correspondingly worsened signal-to-noise ratio (SNR) and/or a worsened dose utilization.
Another possibility for reducing the signal drift lies in the irradiation of the detector material with visible light or with infrared light in addition to or during the irradiation of the detector with X-ray radiation.
Furthermore, the possibility exists that a calibration of an X-ray detector is carried out during irradiation of the semiconductor layer of the X-ray detector. This makes use of the fact that a calibration is made more exactly if it is carried out taking account of the current polarization state of the semiconductor layer of the X-ray detector. This is the case, in particular, if the electrical signals are generated during the calibration process by means of a charge carrier transport in the semiconductor layer. This charge carrier transport is achieved by coupling light into the semiconductor layer wherein charge carrier clouds are generated which resemble those charge carrier clouds which typically arise through interaction of an X-ray quantum of a particular energy with the semiconductor layer.
Another conventional procedure lies in correcting the detector drift with a “pre-scan”. Immediately before the image recording, a counting rate calibration takes place wherein the X-ray detector is irradiated with a dose rate bandwidth by variation of the tube current of an X-ray tube that is used, said dose rate bandwidth corresponding to the dose rate bandwidth to be expected at the X-ray detector during the later image recording.
A further source of measuring errors of X-ray detectors is caused by inhomogeneities in the sensor and the ASIC performing the readout.
In addition, it is also desirable to be able to test an X-ray scanner in advance for its correct functioning.